"It was almost unbelievable."

Wide Magnet

On Christmas Day last year, NASA's MAVEN spacecraft observed something entirely unexpected and exceedingly rare while orbiting Mars.

Specifically, it appears to have noticed the almost complete "disappearance" of solar wind, the steady trickle of charged particles coming from the Sun, in the wake of a powerful solar event, according to NASA.

The sudden lack of pressure from solar wind allowed the Martian atmosphere and magnetosphere to suddenly expand by thousands of miles — more than triple its regular size — catching scientists by surprise.

"When we first saw the data, and how dramatic the drop in the solar wind was, it was almost unbelievable," said University of Iowa Jasper Halekas, lead author of a new study being presented at the ongoing American Geophysical Union Fall Meeting, in a NASA statement.

"We formed a working group to study the event, and we have found this time period to be rich with incredible findings," he added.

Solar Windswept

As a result of being assailed by the Sun's particles, the Martian atmosphere is slowly disappearing, a process known as "atmospheric escape."

On the day of that dramatic solar event in December 2022, much faster-moving particles appeared to leave a void of "extremely low-density solar wind" behind, according to NASA.

That allowed the suddenly unencumbered Martian atmosphere and ionosphere to balloon to more than triple its usual size, causing the ionosphere to become temporarily unmagnetized.

The event was picked up by MAVEN's scientific instruments, allowing scientists to get a better sense of how these processes work.

"We are really getting to see how Mars responds when the solar wind is effectively removed," Halekas said. "It makes for a great outlier study on what Mars would be like if it were orbiting a less ‘windy’ star."

"MAVEN was designed to observe these types of interactions between the Sun and the Martian atmosphere, and the spacecraft provided exceptional data during this truly anomalous solar event," MAVEN principal investigator Shannon Curry added.

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